1. Field of the Invention
The present invention relates to a wire bonding method and apparatus.
2. Prior Art
Manufacturing processes for assembled-semiconductor devices such as ICs, etc. include a wire bonding process. In this bonding process, wires 4 are connected to pads 1a (first bonding points) of a semiconductor chip 1 on a workpiece 3 and leads 2a (second bonding points) on a lead frame 2 as shown in FIG. 12. FIG. 13 shows the wire bonding method used in the above-described wire bonding process, and FIG. 14 shows a wire bonding apparatus 10 for such a process.
In FIG. 13, a ball 4a is first formed in step (a) by the spark discharge of an electric torch 6 on a wire 4 that extends from the lower end of a capillary 5. Afterward, the electric torch 6 is moved in the direction indicated by arrow. Next, in step (b), the capillary 5 is moved to a point above the first bonding point 1a. Then, in step (c), the capillary 5 is lowered, and the ball 4a at the tip end of the wire 4 is connected to the first bonding point 1a. Afterward, in step (d), the capillary 5 is raised. Then, in step (e), the capillary 5 is moved to a point above the second bonding point 2a; and in step (f), the capillary 5 is lowered and the wire 4 is connected to the second bonding point 2a. Subsequently, in step (g), after the capillary 5 has been raised to a fixed position, a damper 7 is closed, and the capillary 5 and damper 7 are raised together, thus cutting the wire 4. One wire connection is thus completed.
Generally, in the above wire bonding method, the deviations of at least two fixed points on the semiconductor chip 1 and at least two fixed points on the lead frame 2 from the regular positions are first detected by the position detection camera 11 shown in FIG. 14, and then bonding coordinates that have been stored in memory beforehand are corrected by a calculating part on the basis of these detected values. In the case of detection by this position detection camera 11, an X-axis motor 12 and Y-axis motor 13 are driven so that the optical axis 11a of the position detection camera 11 is positioned directly above the measurement point. After the bonding coordinates have been corrected as described above, the capillary 5 is moved in the directions of the X and Y axes and the direction of the Z axis, and the wire 4 passing through the capillary 5 is wire-bonded to the first bonding point 1a and second bonding point 2a as illustrated in FIG. 13.
In the above operation, since the optical axis 11a of the position detection camera 11 and the axial center 5a of the capillary 5 are offset from each other by a distance W, the XY table 15 is moved for the offset amount W by the X-axis motor 12 and Y-axis motor 13 after the deviations of the fixed points have been detected by the position detection camera 11 and the bonding coordinates have been corrected, and then the capillary 5 is positioned above the first bonding point 1a. Afterward, the wire 4 is wire-bonded at the above-described corrected bonding coordinates. This is done by the movement of the XY table 15 in the directions of the X and Y axes by the X-axis motor 12 and Y-axis motor 13 and further by the movement of the capillary 5 in the direction of the Z axis by the raising and lowering (or swinging) of the capillary arm 16 caused by the Z-axis motor 14. In FIG. 14, the capillary arm 16 is disposed on a bonding head 17 so that the capillary arm 16 is swingable, and the position detection camera 11 is fastened to the bonding head 17 via a camera holding arm 18. Xw indicates the X-axis component of the offset amount W, and Yw indicates the Y-axis component of the offset amount W.
The size of the ball 4a shown in step (a) in FIG. 13 and the length (tail length) and shape, etc. of the wire 4 extending from the lower end of the capillary 5 shown in step (g) in FIG. 13 constitute information that is important in determining the optimal conditions of bonding. Japanese Patent Application Laid-Open (Kokai) No. 60-242627, for instance, discloses a conventional method and apparatus for detecting the ball diameter, tail length and shape, etc. In this apparatus, the ball or the tail length extending from the capillary is detected by a detection means from the side, and the tail length or the ball diameter is measured based thereupon. In this method, since the tail length and ball diameter are directly measured, the optimal bonding conditions can be determined with good precision.
However, the above-described prior art requires a special detection means for measuring the tail length and ball diameter, etc. Accordingly, the apparatus is complicated in structure, and thus the cost of the apparatus tends to be high.